Device for the electrodeposition of aluminum or aluminum alloys from organometallic electrolytes containing alkyl

ABSTRACT

A device for the electrodeposition of aluminum and/or aluminum alloys from organometallic electrolytes containing alkylaluminum complexes on materials to be coated, said device consisting of a supporting frame with a stand and transportation bearings, at least one plating barrel, at least one drive unit for the plating barrel, and one or more holding arms for the plating barrel, characterized in that the drive unit ( 3 ) is arranged in an encapsulated gas-tight housing, the plating barrel ( 13 ) has a perforated inner tube ( 15 ) arranged along the longitudinal axis thereof and open at its side, so that, when placing the device in an electrolyte container, the lateral openings are arranged directly opposite the electrolyte feed in the electrolyte container, the plating barrel ( 13 ) consists of a material which is stable both in aqueous and organometallic electrolytes at temperatures up to 110° C.

[0001] The invention relates to a device for the electrodeposition of aluminum or aluminum alloys from organometallic electrolytes containing alkylaluminum, said device consisting of a supporting frame with a stand and transportation bearings, at least one plating barrel, at least one drive unit for the plating barrel, and one or more holding arms for the plating barrel.

[0002] Electroplating of small parts and bulk material in aqueous solution, such as nickel-plating or zinc-plating, is usually effected in rotating, perforated barrels made of polyethylene or polypropylene. These barrels are driven by electric motors arranged in a plastic housing in the supporting rack. Current transfer to the goods mostly is effected by means of flexible copper cords arranged laterally on the barrels and enveloped with a plasticized PVC tube to prevent undesirable epitaxial growth of metal.

[0003] Electrodeposition of aluminum or aluminum alloys from aqueous solutions is not possible due to the very low position of the potential of aluminum. Consequently, electrodeposition must be effected from non-aqueous organic systems. In particular, electrolytes containing alkylaluminum are used to this end, with organic solvents normally being employed. Therefore, deposition of finely crystalline aluminum and layers of aluminum alloys is achieved in an excellent fashion from anhydrous alkylorganoaluminum electrolyte systems, the alkylaluminum complexes being dissolved in aromatic hydrocarbons such as toluene.

[0004] However, the plating barrels employed in aqueous electroplating cannot be used in organic electrolyte systems. This is connected with the organic solvents being used and with the operating temperatures of from 90 to 100° C. where such electroplating is carried out. At such temperatures and in the corresponding organic solvents, conventional barrels for aqueous systems are not stable, undergoing decomposition or dissolution, and thus may contaminate the electrolyte. Furthermore, there is a risk of distortion of the barrels to such an extent that mechanical stability is no longer guaranteed.

[0005] Also, electroplating systems for bulk material which are used in organic media, particularly for the deposition of aluminum, are known from the prior art. However, these systems failed to gain general acceptance in practice.

[0006] This also includes the state of the art described in EP 0 042 503 A1. Therein, a device for the electrodeposition of aluminum from organic electrolytes has been described. The aim of the above invention is to create a device where the plating barrel is not required to be removed from the plating trough for loading and unloading. The above state of the art describes the use of a conveyor means for the parts to be coated, which is used to fill the plating barrel and extends via a gate into the interior of the plating trough, terminating above a closable opening of the plating barrel. The barrel can be opened and closed from outside, and, in order to empty the barrel, a discharge container exposable to inert gas and inert fluid is provided, which is arranged beneath the plating trough and is connected with same via a tube-shaped connector element which can be shut off.

[0007] The above state of the art represents a highly complex construction of a plating barrel which failed to gain general acceptance in practice as yet.

[0008] The invention is based on the object of providing a device for the electrodeposition of aluminum from organic electrolyte systems, in which device the plating barrel is modified in such a way that the plating barrel is stable in the media employed and at the temperatures applied, has a safe drive in flammable media, and nonetheless allows high-quality coating with aluminum or alloys thereof.

[0009] Said object is accomplished by means of a device wherein the drive unit 3 is arranged in an encapsulated gas-tight housing, the plating barrel 13 has a perforated inner tube 15 arranged along the longitudinal axis thereof and open at its side, the lateral openings being arranged directly opposite the electrolyte feed in the electrolyte container, and the plating barrel 13 consisting of a material which is stable both in aqueous and organometallic electrolytes at temperatures up to 110° C.

[0010] By encapsulating the drive unit in a gas-tight housing, driving the barrel in flammable liquids is made much safer. The case preferably consists of stainless steel, and the drive shaft for the barrel is conducted through the housing wall by means of a gas-tight shaft guide with a sealing, preferably one made of polytetrafluoroethylene.

[0011] To protect the drive motor, and as an additional safeguard against penetrating flammable organic solvents, the housing case is flooded with an inert gas such as nitrogen or argon and provided with an overpressure of preferably 0.1 to 0.3 bars. The housing is also equipped with a feed valve and an overpressure blow-off valve with a nonreturn flap.

[0012] In each loading/unloading procedure, inert gas at a pressure of about 0.1 to 0.2 bars above the set value of the blow-off valve is automatically fed via the feed valve into the drive unit housing on the station. Following each coating process, the inert gas atmosphere in the drive unit housing is purged, and the overpressure in the housing is reset after each cycle. The purging time or the amount of inert purge gas is set via the plant controls.

[0013] Another problem of the plating barrels known from the prior art is the stability of the barrel material. In the long run, conventional barrel materials such as polyethylene and polypropylene are not stable in the organic solvents used in aluminum coating.

[0014] This problem is solved by using suitable plastics insoluble in organic solvents and reinforced with fiberglass. In a preferred fashion, the plating barrels are produced from at least glass-fiber reinforced polyphenylene sulfide including a proportion of glass fiber of at least 40%. This ensures chemical stability of the plating barrels at operating temperatures in the electrolyte of up to 110° C., as well as abrasion resistance.

[0015] In a preferred embodiment, the drive gearwheels are made of the same material. Another advantage is that this material is also stable in dilute acids and bases, so that pretreatment and secondary treatment of the parts to be electroplated can be effected in aqueous systems such as acids and/or bases in the same barrel without transferring.

[0016] Providing the plating barrel with a perforated inner tube results in an improvement of electrolyte circulation. In electrodeposition of metals from organic electrolytes, the electrolyte circulation plays an exceptionally important role because, as a result of the limited solubility of organometallic complexes, depletion of metal ions in the liquid boundary layer near the product may rapidly occur in case of insufficient electrolyte circulation. This results in quality losses in the coating of the materials, especially in burning of the materials to be coated, in rough and uneven layers, and possibly even in electrolyte decomposition. In particular, this problem arises in alloy deposition of aluminum, but is also observed in pure aluminum deposition. To avoid this problem, the inventive device in the barrel is equipped with a perforated inner tube which is arranged along the longitudinal axis of the plating barrel and has lateral openings facing the container wall of the electrolyte container. When placing the device of the invention in the electrolytic bath, the lateral openings of the inner tube are situated directly opposite the electrolyte feed lines in the container wall. In this way, pumping of fresh electrolyte at high speed through the inner tube and directly to the substrate is accomplished during coating, so that good exchange is ensured and the drawbacks described above are prevented from occurring. In a preferred embodiment, it is also possible to arrange an additional auxiliary anode in the inner tube, thereby further augmenting the local concentration of metal ions and increasing the coating rate.

[0017] According to the prior art, the holding arms of conventional barrels are mostly rubber-coated and thus unstable in organic electrolytic baths. This also applies to the conventional PVC jackets of electric conductor tracks for the electrolyte current. When using such an arrangement, epitaxial growth of metal on the power supply bars is therefore to be expected. According to the invention, this problem is solved in that the holding arm in the form of a hollow body consists of steel and has a core of polyphenylene sulfide. Arranged in this core of insulating material is the power supply bar for the power supply of electrolysis. It is only inside the barrel where connection between the power supply bar and the contact bulb in the product is made in the bearing of the holding arm. Owing to this type of construction, additional protection of the power supply bar against undesirable epitaxial growth of metal is no longer necessary. The holding arm itself has no electric potential applied thereto and is additionally protected on the outside by a plastic layer coated thereon, preferably one made of PVDF (polyvinylidene fluoride) or of thermoplastic fluorocarbons based on ethylene and chlorotrifluoroethylene.

[0018] The invention will be illustrated in more detail with reference to FIG. 1 below. The numeral 1 designates the supporting frame with stand, which includes the single elements of the device, the plating barrel, the drive unit, and the holding arms. The supporting frame has transportation bearings 4 arranged thereon which are used to lower or lift the device into or out of the respective electrolyte or rinsing baths.

[0019] The supporting frame has the drive motor 3 encapsulated therein, which is suspended so as to be electrically insulated and has a gas-tight shaft guide 5. A drive gearwheel 6 preferably made of polyphenylene sulfide is arranged at the end of the shaft. The drive gearwheels drive the plating barrel 13 which preferably consists of glass-fiber reinforced polyphenylene sulfide. The plating barrel 13 is connected with the supporting frame 1 via the holding arms 11. The holding arms 11 are preferably made of stainless steel, they are hollow and coated with fluoropolymers on the outside thereof. The hollow space of the holding arms 11 includes an insulating material wherein the power supply bars 9, 10 for the electrolysis power supply are arranged. The numeral 12 designates the bearing block for the plating barrel. The plating barrel has perforated side walls 14 and a perforated inner tube 15 which is open at its side. An inner auxiliary anode 17 can be introduced into the barrel through this tube so as to achieve higher electrolyte concentrations near the material to be coated. The numeral 18 designates the pick-up contacts arranged in the barrel, which preferably consist of copper. Furthermore, flexible current transfer contacts 16 are situated inside the plating barrel.

[0020] The numeral 9 designates the power supply line for the material to be coated, which line is insulated inside the barrel holders. The numeral 7 designates the inert gas vent of the drive unit housing, including a nonreturn flap.

[0021] Using the device according to the invention, it is possible to produce high-quality coatings of aluminum or aluminum alloys. Coatings of magnesium and magnesium alloys are also possible, in which case the corresponding alkymagnesium-containing electrolytes are employed. The device of the invention is hard-wearing and can also be used in aqueous systems, e.g. in rinsing procedures.

[0022] Reference List

[0023]1 stand

[0024]2 power supply bar

[0025]3 encapsulated drive motor with electrically insulated suspension

[0026]4 transportation bearings

[0027]5 gas-tight shaft guide

[0028]6 drive gearwheels

[0029]7 inert gas vent of motor housing with nonreturn flap

[0030]8 inert gas purge valve

[0031]9 power supply line for material to be coated, insulated inside barrel holder

[0032]10 insulating material inside holder

[0033]11 barrel holder made of stainless steel, coated with pvdf/halar on its outside

[0034]12 bearing block for barrel

[0035]13 barrel made of glass-fiber reinforced pps

[0036]14 perforated side walls

[0037]15 perforated inner tube

[0038]16 flexible current transfer contacts

[0039]17 inner auxiliary anode

[0040]18 pick-up contacts 

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 9. A device for the electrodeposition of aluminum and/or aluminum alloys from organometallic electrolytes comprising alkylaluminum complexes on materials to be coated, said device comprising of a supporting frame with a stand and transportation bearings, at least one plating barrel, at least one drive unit for the plating barrel, and one or more holding arms for the plating barrel, wherein at least one drive unit is arranged in an encapsulated gas-tight housing, the plating barrel comprises a perforated inner tube arranged along the longitudinal axis thereof having a side opening such that when placing the device in an electrolyte container, the side opening is arranged directly opposite an electrolyte feed in the electrolyte container, and the plating barrel comprises a material which is stable both in aqueous and organometallic electrolytes at temperatures up to 110° C.
 10. The device according to claim 9, wherein the plating barrel comprises polyphenylene sulfide reinforced with at least 40 wt. % glass fiber.
 11. The device according to claim 9, wherein the drive unit is provided with an automatic purging and pressurizing device for inert gas.
 12. The device according to claim 9, wherein an auxiliary anode is arranged in the inner tube in order to increase the concentration of metal ions.
 13. The device according to claim 9, wherein the drive unit has a drive shaft which is conducted through the housing wall of the drive unit via a gas-tight shaft guide comprising polytetrafluoroethylene.
 14. The device according to claim 9, wherein the holding arms are hollow bodies made of steel, comprising a core of polyphenylene sulfide.
 15. The device according to claim 9, wherein the holding arms on the outside thereof are coated with a plastic layer of polyvinylidene fluoride or thermoplastic fluorocarbon based on ethylene and chlorotrifluoroethylene.
 16. The device according to claim 14, wherein the power supply bars for the electrolysis power supply are arranged in the core of polyphenylene sulfide. 